This invention relates to a lubrication system for motorcycles, and more particularly to a dry-sump type lubrication system wherein the forward half section of the frame unit of a motorcycle is concurrently used as a lubrication oil tank.
The oil tank of the prior art lubrication system is defined by the internal spaces of the mutually connected main pipe, down tube, and tension pipe constituting the forward half section of the frame unit. Since, however, the upper end of the down tube is closed by being connected to the steering head pipe, air remains in the upper end region of the down tube when lubrication oil is brought into the oil tank through an inlet, reducing the oil capacity of the tank. Air bubbles contained in the lubrication oil held in the down tube increase the volume of air remaining in the upper end region of the down tube to obstruct the supply of lubrication oil to the engine. Further, air is conducted together with lubrication oil to those parts of the engine which have to be lubricated, often causing said part to be short of an oil film, and consequently giving rise to the wear and seizure of said oilless parts.
The prior art lubrication system wherein a breather intended to compensate for fluctuations in the air capacity of the lubrication oil tank is open to the atmosphere has various disadvantages that lubrication oil evaporates to the outside through the breather to be quickly depleted; when a motorcycle is thrown down, lubrication oil flows out of the breather; and dirt and dust are carried into the lubrication oil tank through the breather to accelerate the contamination of lubrication oil. Since the rear half section of the frame unit is directly welded to the forward half section thereof which is concurrently used as a lubrication oil tank, leakage, if any, of lubrication oil from said tank has to be examined only after the whole frame unit is fully constructed, rendering said examination considerably difficult due to the bulkiness of the frame unit thus constructed.